Negative price means they're paying you to take the product. They're not paying you to
do something unless doing that something is the product.
In the case of negative-priced airline tickets, they are not paying you to ride an airplane. They're paying you to take a ticket. That ticket then gives you an option to ride an airplane. Ticket use is not mandatory, not even ethically.
If it were somehow advantageous for the airline to actually have people take the trip from B->C, then they could pay people to actually do it by, for example, giving you a rebate when you step off the airplane at C, or charging you a fee for not boarding the airplane at B. But they don't do this. So either they benefit from just selling the ticket, or far more likely they don't benefit from selling the ticket but are trying to manipulate the system in some other way.
I don't understand your optimization comment. There is no scenario in which it's cheaper to fly more people from B->C than fewer. Larger, more efficient planes are still more expensive in total to operate, so if your passengers fit on a smaller plane you'll save money that way, not lose money. Even if the bigger plane were somehow cheaper in total, nothing says you need to fill it up. You can just fly your normal B->C passengers in a bigger plane with lots of empty seats, if that ends up being cheaper. Airliners don't require ballast. There is no scenario where the airline makes more money at the end of the day by having you use your B->C ticket compared to obtaining a B->C ticket but not using it.
I also don't understand your "flying privately" comment. Airlines are cheaper than the alternative, so it's OK if they play pricing games? Well, it's a free country, they can play as many pricing games as they want, but I'm going to play them too.
